Double layer in ionic liquids: Overscreening vs. crowding
Abstract
We develop a simple Landau-Ginzburg-type continuum theory of solvent-free ionic liquids and use it to predict the structure of the electrical double layer at an electrode. The model incorporates effects of overscreening from short-range correlations, dominant at small voltages, and steric constraints of finite ion sizes, which prevail at large voltages. We show how increasing the voltage gradually suppresses overscreening in favor of the crowding of counterions in a condensed inner layer near the electrode. The corresponding modes of capacitance-voltage dependence predicted by the theory are consistent with recent computer simulations and experiments on room-temperature ionic liquids, using a correlation length of order the ion size.Keywords
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